‘Significant’ air quality benefits from green walls

A massive 30% reduction in air pollution could be achieved by ‘greening up’ city streets, according to research published today (July 18) in the Environmental Science and Technology journal.

Scientists at the Universities of Birmingham and Lancaster (UK) have found that trees, bushes and other greenery growing in the concrete-and-glass ‘urban canyons’ would deliver cleaner air at the roadside where most of us are exposed to the highest pollution levels.

The green wall recently installed by TfL close to the Mermaid theatre in Blackfriars

And, they found that this can be implemented on a street-by-street without the need for large-scale and expensive initiatives.

Plants in cities clean the air by removing nitrogen dioxide and microscopic particulate matter, both of which are harmful to human health. These pollutants are significant problems in cities in developed and developing countries. For instance, the Environmental Audit Committee estimates that outdoor air pollution causes 35,000-50,000 premature deaths per year in the UK.

Canyons

The researchers have found that, because pollution cannot easily escape street canyons, ‘green walls’ of grass, climbing ivy and other plants have a better opportunity than previously thought to act as an air pollution filter. Instead of reducing pollution by 1 or 2%, reductions of more than ten times this magnitude could be achieved, according to this study.

Using a computer model that captures the trapping of air in street canyons, as well as the hundreds of chemical reactions that can affect pollution concentrations, the research team could distinguish the effects of plants in canyons from those of plants in parks or on roofs. Green walls emerged as clear winners in terms of pollutant removal. Street trees were also effective, but only in less polluted streets where the tree crowns did not cause pollution to be trapped at ground level.

The researchers even suggest building plant-covered “green billboards” in these urban canyons to increase the amount of foliage.

For the first time, the study predicts that a significant effect on pollution could be achieved on a street-by-street basis.

Professor Rob MacKenzie, from the University of Birmingham, told AirQualityNews.com: “The amount of pollution that plants can remove from street canyon air is much greater than previously thought – as much as ten times greater. The impact depends on how much greenery. The number we quote in the abstract is up to 40% for nitrogen dioxide and 60% for particulate matter.

“It is substantial especially when you consider all that is being done to remove concentrations at urban roadsides – such as scrapping old cars, adding catalytic converters to cars, and bringing in the congestion charge – have not all produced the results needed.”

He added: “The benefit of green walls is that they clean up the air coming into and staying in the street canyon – planting more of these in a strategic way, could be a relatively easy way to take control of our local pollution problems.”

Dr Tom Pugh, from Lancaster University, said: “More care needs to be taken as to how and where we plant vegetation in our towns and cities, so that it does not suffer from drought, become heat stressed, vandalised, or interact negatively with other aspects of our urban areas, and can carry out the very important job of filtering our air.”

TfL

The research was welcomed by Transport for London (TfL), the local government body responsible for most transport in the capital.

Two weeks ago, TfL was criticised for investing in green walls rather than tackling emissions at source (see AirQualityNews.com story).

Nicola Cheetham, head of environment at TfL, said: “I am pleased that this research adds to our knowledge of the benefits of green infrastructure and the optimum locations for placing green walls in transport corridors.

“The publication of the results coincides with the completion of TfL’s second green wall at The Mermaid, Blackfriars, installed as part of the Clean Air Fund programme. Our own research, conducted by Imperial College London, shows the ability of different plants to trap particulate matter. The bringing together of these various strands of research will help to inform the planners, designers and green infrastructure professionals who are responsible for the provision and management of green infrastructure in our towns and cities.”